Method of casting rotors



9, 1966 E. R. SUMMERS ETAL 3 METHOD OF CASTING ROTORS Onginal Filed Jan. 6, 1960 5 Sheets-Sheet 1 IN VEN TORS Eawm R. surmcns eaoma m. aossuamaxm ATTORNEY 9, 1966 E. R. SUMMERS ETAL 3264,95

METHOD OF CASTING ROTORS 5 Sheets-Sheet 2 Original Filed Jan. 6. 1960 INVENTORS ERWI N R. SUMMERS GEORGE M. ROSENBERRY, JR.

ATTORNEY FIG. 6

g- 9, 1965 E. R. SUMMERS ETAL 9 METHOD OF CASTING ROTORS Original Filed Jan. 6, 1960 5 Sheets-Sheet 3 3 INVENTORS ERWIN R.SUr1MERS GEORGE M. ROSE NBERRXJR;

ATTORNEY 9, 1966 E. R. SUMMERS ETAL 2 METHOD OF CASTING RQTORS 5 Sheets-Sheet 4 Original Filed Jan. 6. 1960 a M mm EB MN mmfl Wwm I. n N a mm a ATTORNEY 1966 E. R. SUMMERS ETAL. 3,264,6Q5

METHOD OF CASTING ROTORS Original Filed Jan. 6. 1960 5 Sheets-Sheet 5 l w Q m a :2 24 CURRENT now I32 '3 CURRENT FLOW IN V EN TORS ERWIN R. SUMMERS GEORGE N. ROSENBERRY, JR

BY A Q I 5 ATTORN Y United States Patent 3,264,695 METHOD OF CASTING ROTORS I Erwin R. Summers, Scotia, and George M. Rosenberry, .lr., Schenectady, N.Y., assignors to General Electric Company, a corporation of New York Uriginal application Jan. 6, 1960, Ser. No. 857, now Patent No. 3,213,306, dated Oct. 19, 1965. Divided and this application Jan. 28, 1965, Ser. No. 436,979

1 Claim. (Cl. 22-200.5)

This application is a division of our copending application Seri-al No. 857, filed January 6, 1960, now US. Patent No. 3,213,306, issued October 19, 1965.

The invent-ion described herein relates to dynamoelectric machines and more particularly to a method of castring high resistance rotors, including an improved design of winding for securing a high degree of electrical performance during operation.

High resistance motors of the type classified as NEMA design D, normally are .used in applications requiring high torque at starting and during acceleration; or where very high loads are suddenly applied when the motor is running at full speed, as in a punch press drive for example, and it is desired to have the motor electrical torque increase relatively slowly as the speed is reduced, such that the inertia, or a portion of the kinetic energy, of moving parts such as a flywheel can 'be utilized to cushion or reduce the peaks of electrical current and power input to the motor. These high slip rotors usually are made of preformed conductor bars or high resistance alloys which are cast in the conductor slots. Such cast alloys having suitable properties for windings, are presently limited to resistivities only moderately higher than aluminum, such that the con-ductor size and thermal inertia are relatively small and the thermal resistivity is correspondingly high, thereby making it difficult to dissipate heat to the surrounding iron, or to conduct heat axially along the winding to the end portions of rotor for transfer to ventilating air or other media.

It is generally recognized that small bar sections have the disadvantage of low thermal storage in addition to being expensive to construct and install in the rotor slots. The drawbacks of small bar sections have been successfully overcome by the construction disclosed and claimed in US. Patent 2,767,340 issued to Walter J. Martiny, Jr., and assigned to the same assignee as the present invention. The Martiny construction of having discontinuous high heat storage sections extending from the axial conductors toward the rotor bore permits adequate dissipation of heat generated in machines up to about 30 HP. Beyond that range, the construction is not too successful in transferring and dissipating heat from the rotor in quantities sutficient to obtain optimum performance in the NEMA design D motors having the higher slip ratings, such as 15% decrease in speed from no load to normal full load torque, or to provide suitable thermal storage capacity for stalled rotor or delayed acceleration.

Windings with indirect or elongated conductor paths of greater length than the punching stack, sometimes called a zigzag pattern, and of other configurations used for increasing the length cross section area and thermal storage capacity of the conductor path have been suggested,

Patented August 9, 1966 and although they have certain desirable attributes, acceptable casting processes have not been developed so that windings of this type have not been used as a practical matter in high slip rotors.

The problems associated with casting high resistance squirrel cage rotors is sizes greater than about 30 HP. are extremely difiicult to overcome. Pressure casting techniques are used successfully in small rotors and in carrying out this process, aluminum is cast at a high pressure such as 6000 p.s.i. thus compressing the trapped air to about 4 or 0.25% of its initial volume, and the slots are, for all practical purposes, completely filled with aluminum as desired.

As now practiced however, pressure casting is not generally used with large rotors especially those having large end rings of the type used in two-pole motors. The twopole end rings are large as compared to the etfective conductor area of a slot or to the area of a sprue gate normally used in the pressure casting process. The molten aluminum freezes in the smaller sections of such a pressure cast winding before the complete winding can be solidified and as a result, large shrink holes are likely to be established in the end rings which may effectively interfere with dynamic balance and rotor current distribution. Also, because of the great size and uneconomically high cost of a massive pressure casting machine for large rotors, it becomes desirable to be able to cast such large rotors with less rugged equipment and at low pressures such as are used in the centrifugal casing process.

Consideration of centrifugal casting methods of the type disclosed and claimed in the copending patent application to E. R. Summers, Serial No. 681,519, filed September 3, 1957, and entitled, Method of Casting Squirrel Cage Rotors, now US. Patent No. 2,976,773 issued August 22, 1961, leads one quickly to the conclusion that a centrifugal method that is satisfactory, for casting metal in straight slots can be unsuccessful for casting conductors in zigzag slots or in slots having heat sinks of the Martiny type. When a rotor having slots of zigzag or other configuration is rotated about the vertical axis of the lamination stack and aluminum introduced therein, it will be seen that aluminum flows axially downward or upward through the conductor sections at theoutermost portions of the slots, and the centrifugal force resulting from rotation is effective in keeping the aluminum in this zone of the slots. The relatively lighter air remains trapped at the smaller radius of rotation in the pockets at the bottom or inner portions of the slots, and can neither escape radially between the laminations of the compressed stack nor axially along the slots because of the barriers interposed by the flopped punchings which are used for obtaining the slots of zigzag configuration. Since very little liquid metal pressure is developed during centrifugal casting, this trapped air is not compressed appreciably, the aluminum does not fill the bottom portion of the slots and the desired thermal characteristics of the winding are not realized.

In view of the foregoing, it is evident that neither conventional pressure nor centrifugal casting methods can be used for producing acceptable windings in all sizes and types of high-slip rotors. Pressure casting methods create shrink holes in the larger end rings, because the aluminum freezes in the slots before the complete winding can become solidified. A sprue and gates of suitable size cavities.

Therefore, the primary object of our inventionzis to provide an improved casting method for providing com: pletely' for-med windings including sound end rings and conductors in the rotor slots of high resistance rotors.

Another object of our invention is the provision of a new squirrel cage winding for permitting improved per formance of high resistance rotors.

In carrying out our invention we select a plurality of punchings, having slots of different configuration, and stack them in sections to form a rotor core for receiving a high resistance winding. Certain of the sections are separated by spacers at spaced points along the core length to form openings leading into the bore. During casting, the rotor is rotated and a molten metal or alloy is poured downward through a funnel in the bore and thrown outwardly by centrifugal force through radially extending gates in the bottom mold to fill the lower fan blade and end ring cavities and commence rising in the slots. Air displaced by the molten mass flows upwardly throughthe slots, inwardly through vent openings at the bore, and also from the slots through gates at the top mold. As the molten mass rises in the bore, the vent openings as they become submerged are successively utilized as a means for feeding molten ma terial from the bore into thevslots for assuring a sound casting in these areas. Gates 'in the upper mold facilitate casting of the upper fan blade and end ring cavities. Since it is necessary to preclude establishment of" shrink holes. in the various parts of the winding, the rotor bore is:

filled to the top of the mold and as the complete winding freezes, a quantity of molten aluminum is available to augment the aluminum in the windingas shrinkage oc-. curs during freezing. The funnel is withdrawn before I the sprue freezes in the bore. It will be apparent to those skilled in the art that the provision of this improvedcasting process provides new freedoms for designing slots for windings capable of more rapidlydissipating heat to zones 7 of lower temperature in the rotor.

The subject matter which we regard as our invention is particularly pointed out and distinctly claimed in the, concluding portion of this specification; Our invention K however, both as to organization and method of operation, together with further objects and advantages thereof,

may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

FIGURES 1a, 1b, and 1c are views in elevation of a punchings either of which may be combined with the punchings ofFIGURES la-lc to form a rotor core;

FIGURE 6 is a view in elevation, partly in section, showing a portion of rotor winding which utilizes the punchings of FIGURES a and 5b;

FIGURE 7 is a sectional view in elevation of a portion of a rotor illustrating the arrangement of laminations used for obtaining less generation of heat at andbetter heat flow away from the center part of the rotor; and

FIGURE 8 is a view similar the laminations are designedina different manner for providing a short straight-sectionof conductor located in thecenter part of the rotor.

Referring now to the drawings wherein like. reference characters designate like vor corresponding, parts throughout the severalviews, there .is'shown in FIGURES 11alc, three separate-designs of silicon or other types of steel laminations or punchings. It :will be understood that although only a .few.slots are shown in each lamination,

the slots are equally spaced and extend completely around the punchingin the sameconventional manner as that known in the prior art. a In FIGURE la,'the lamination 20. has a central bore 21=and a bridge 22 between the outer diameter and a plurality of slots 24, eachof which includes a neck 26 for thermal storage and a conductor section 28forconduction; of current.

The; lamination 30=ofFIGURElb has a central'bore A 31, is of the same outer diameter, as lamination 20=and is equipped with a bridge 32 of a radial length equal to bridge 22.. The slots 34 and.36 however are of a different Slots 34 comprise circular openings which configuration. constitute the main conductor section whileslots36 are used as vents and/ or gates; in the bore as described more fully hereafter.

The lamination 40 of FIGURE lc, likewise has the same size bore 41 and outer diameter and bridge 42 as punchings'20fand 30 0f FIGURES'la and lb. Howevenrit is v but in manyinstances it is desirable to have them open to the rotor surface. Casting problems are. not encountered-because a hardenable asbestos typev of putty I can: be packed in the surface openings for preventing escape ofmolten'metal when theawinding is cast.- A

particular benefit achieved is that the openings can be;

used for. aligning the .laminations, especially in. rotors des1gnedfor a zigzag winding; since they do not contain uninterrupted axial slot .openingsg as in most rotors. When the slotis buried vdeep in the la'rninations, index markmgscan be punched in the; outer pe1ipheral:surface directly opposite the slot for facilitating alignment.

The castingapparatus shown is described more specifically :in the above mentioned copending patent application. Referring specifically to FIGURE 3, the apparatus shown comprises upper and lower molds Sit and.52'

enclosing the rotorcore 54 in :the manner shown. Upon insertion of the rotor core within the molds, hydraulically actuated. arms. 56 attached to. the apparatus are used for drawing the molds toward one another and thereby effectively compressing the rotor core positioned .therein.=

To obtain rotation and consequent-displacement of aluminum radially outwardly from the bore and into the conductor slots, the apparatus is placed on a turntable 58 and rotated by an appropriately sized motor. Both the upper and lowermolds 50 and52 are equipped with fan blade and end. ring cavities 60 and 62 which are connected to the bore by gates 64 formed by circurruferentially spaced slots cut in an annular member in the mold,

Obviously, fan blade? cavitieswill not be used where the rotor isr-of a type where'fan blades cannot conveniently be cast or used with the winding. A venting arrangement 6S is located in the upper mold for completely venting air from the upper end ring cavities when .the winding is cast in the rotor. As shown, a funnel 70 is used for introducing molten aluminum intothe rotor bore and is to FIGURE 7 except that;

designed to have its discharge end terminate at a position within the rotor bore below the lowermost radial vent in the lamination stack. The bottom of the funnel extends below the lowermost radial vent opening to the bore, such that, by virtue of the paraboloid of fluid revolution, the air can pass radially inward through said vents as the molten aluminum fills the corresponding pockets in slots before entering the radial vents communicating thereto. In order to permit venting of air past the funnel during the casting process, spaced spacers 72 are afiixed to the outer periphery of the funnel for permitting air to ascend from the rotor and mold cavities and past the funnel prior to discharge from the mold apparatus.

Referring to FIGURES 2 and 3, the conductor sections corresponding respectively to apertures 28, 34 and 44 have the same slot pitch A (FIGURES lalc) and are co-extensive throughout the length of the rotor. The punching stack is welded along the inner diameter axially between the vents and across spacers 78 for holding the laminations together. When the winding is cast, the aluminum in this area constitutes the elfective current carrying portion of the winding. The yoke Z of lamination 39 overlaps a part of neck 26, thus leaving an area designated 46 equal to the difference between R and R through which aluminum can obtain access to the vent slots 36 during casting. Lamination 40 is equipped only with the apertures 44 and serves to interrupt the neck at interval L, thereby making the bottom portion of the slots discontinuous with respect to current flow. An alternate method Otf providing vents 36 is to weld or otherwise attach spacers 78 to punching 40 within radius R as shown dotted in FIGURE '10, and to make the continuous bore of punching '30 at R instead of R in which case the number of spacers 78 may be less than the number of slots 44. It is evident that the size of the vents at the bore may be varied by using a diiferent number and size of laminations 30 and spacers in each group.

During casting, the mold apparatus is rotated and molten aluminum poured downwardly through funnel 70 into the bore. Centrifugal forces move the molten metal outwardly through the lower gates 64 in the mold where it fills the lower fan blade and end ring cavities 60 and 62. Air therefrom is displaced upwardly through the slots 44, followed by aluminum which fills the individual necks 26 and integrally joined conductor sections 28 in each slot 24, which are equally spaced circumferentially around the rotor .core. Since the molten aluminum assumes a paraboloid of revolution as shown by the concave top surface of the molten aluminum in FIGURE 3, the aluminum will flow through conductor slots 44 in the next group of punchings before the necks in the first section are filled. The slots 36 or alternative spacers 78 in laminations 30, provide vents which serve to allow air to escape from the neck portion of the slot into the bore prior to flowing upwardly past the tunnel for discharge from the mold apparatus. This process of filling the slot portions with molten metal along the rotor core length is continued until the upper end ring and fan blade cavities are completely cast, whereupon casting is continued to provide a molten mass having its upper end positioned above the level of the fan blades and end ring in the mold. The funnel 70 is then withdrawn from the mold apparatus before the molten sprue mass freezes in the punching bore.

As the poured casting freezes, shrinkage in the aluminum occurs and in order to prevent establishment of shrink holes in the various parts of the winding, the sprue provided in the rotor bore serves as a reservoir which continues to feed aluminum into those parts where shrinkage occurs and thereby provide a casting which is sound in all respects. It is evident that the necks of the various conductor slots may develop shrink holes unless they are fed with aluminum. The vents provided along the bore length are therefore subsequently used as gates which serve to augment the aluminum initialy cast in these necks. The mold apparatus is rotated continuously and for a period of time suflicient to permit freezing of the aluminum in the winding and when freezing is accomplished, the molds are then removed from the rotor core and the sprue thereafter removed as described in the above copending application. Difficulty in sprue removal is not encountered because the wall sections forming the gates at either end are accessible and can be cut easily. The aluminum connecting the vent openings with the sprue are of such small cross section that they easily break and do not strongly bind the sprue in the rotor bore.

It is evident that in those situations Where it is desirable to have the vents serve only a venting function, they accordingly will be made sufliciently small so that air can be vented into the bore without permitting the passage of molten material through the vents. Obviously, when used as a combination vent and gate, the vent openings are made of a size sufficient for aluminum freely to pass from the bore into the neck area of the slots. When the combination vent and gate arrangement is used, static casting methods may be employed for casting the winding, especially that method where the cast material is introduced from beneath the rotor.

Many different punching designs may be used for providing a neck portion in the slots which serves as a thermal storage area or heat sink and as vents and/or gates for facilitating casting of the winding. As illustrated in FIGURES 4a and 4b for example, two difie-rent kinds of laminat-ions are used for forming the same construction illustrated in FIGURE 2. In this arrangement, the lamination 30 of FIGURE 4a is equipped with inner and outer diameters R and R and has equally spaced slots 82 disposed circumferentialy around the punching. Each slot consists of a curved neck portion 84 and a conductor section 86.

The lamination 90 of FIGURE 4b is of the same inner and outer diameter, has conductor sections 92 spaced around the lamination and is equipped with open ended slots 94 which serve as vents when the laminations are assembled to form a rotor.

In this arrangement, the bottom of the neck 84 is displaced slot pitch (A/4) circumferentially from the center of conductor section 86. The vents 94 of punchings 90 are displaced a like amount from the conductor sections 92 as shown, with the vents overlapping the bottom portions of the necks by a distance R less R When punching 90 is rotated about axis X-X, the openings illustrated by dotted lines then fall midway between the necks of punching and the bottoms of the slots are interrupted at intervals of L along the stack in the same manner as that previously described.

If the yoke section Y of punchings 20 and 80 are made equal to the yoke section Z of punchings 30 and 90, the permeance of the two kinds of punchings will be essentially equal, no unusual distortion of flux fields will occur and the entire length of the punching stack is effectively utilized. The desired torque characteristics and the effective conductor section-area for current flow correspond essentially to the area only of the continuous conductor section at the top of the slots; whereas the heat storage capacity of the complete winding and the rate of heat transfer to larninations correspond to total slot volume, i.e., conductor sections plus neck portions, and to the total slot perimeter, respectively. Small conductor sections are required to obtain the desired torque, but large masses of material in the complete slots and large surface areas are needed to' store and to dissipate the large amount of P loss corresponding to high torque and high slip. The construction described above effectively permits the transfer of large quantities of heat from the casting while still providing highly desirable torque characteristics.

The above described constructions are primarily used for the purpose of providing a heat sink for and better dissipation of heat generated in the rotor ClUI'lllg'OPeI?" Another fundamental method of increasing the rotor resistance is that of increasing the length of the con-' duction path so that it effectively is much longer than" the core stack length. This isaccomplished in the invenation.

tion by utilizing the laminations illustrated in FIGURES 1a, 1b and 10 above andcombining with them the lami-w nation illustrated in FIGURES a or 5b; As shown,'the

lamination 106 is substantially the same as that .illus-: trated in FIGURE lb except that it does not include the circular conduction section openings 34. It has the same inner and Outer diameter and is equipped with'vent slots 192, similarto 36,,which open into the bore of the lamination. The lamination 110 in FIGURE 5b is similarto that illustrated in FIGURE 5a except the conductor open-1 ing 112 does not extend to the bore of the lamination. The dimensions R through R in FIGURES 5a and 5b are thesame as that for the lamination FIGURE 1b.

Referring to FIGURE 6 it will be seen that an increase in length in the conductor path is made possible by the insertion of laminations 1.00. If lamination 110 is..sub-, stituted for 100, the vent 102 .is eliminated but vent 36 is retained. By making this modification with either lamination 100 or 110 it will be seen that the current now i is required to traverse the path illustrated by the dotted lines in FIGURE 6. vDuring casting, the molten aluminum flows through openings 44 and fills the, first sec:. tion 104 of the rotor, displacing air through vents 102 and 36 during the casting process. Since air is then not available to trap, exclude or interfere with the molten.

mass in the various sections, the metal is then'permittedj to flow freely into section ltl and displace air upwardly andinwardly through the sections positioned thereabove,.,

and through the vents which communicate with the rotor bore. two of these laminations 10th or 119 at each location so that the main flux will not by-pass the conductor and result in high leakage reactance, low torque, and low. power factor. By reference to FIGURE 6 it will be seen that the path for current flow is made considerably longer,

than the core stack thus making it easy to obtain a high r resistance in the winding without utilizing special ma terials and still maintain a large value of thermal capacity.

A particular advantage derived from the use, of a construction permitting venting to the bore along the stack length lies in the many different designs of conductor sec-, tions and necks which constitute the heat sinks that may. be used in squirrelcage windings, The parts of the rotor core from which itis most difficult to dissipatejheat are located at the middle of the punching stack. The con-. struction shown in FIGURE 7 is used for obtaining a smaller 1 loss in the middle area .by using a graduated zigzag conductor having a larger cross section for radial current flow at the middle 128 ratherthan at the ends 122 '1 This is accomplished by spacing the barrier of the rotor. laminations 124 a greater distance in the central portion of the core than those at therotor ends. Since there is less resistance to current flow in the middle, the amount of heat generated is not as high and a corresponding reduction in the temperature gradient therefore takes place. Since a larger percentage of total heat loss will occur in the end .portions of the slots, the heat can be dissipated more readily to the fan blades and end rings and then be eifectively carried away by ventilating air. of FIGURE 7 show current flow. The construction shown in FIGURE 7, which does not illustrate venting to the bore, would be suitable for some arrangements of static casting, and also for pressure casting if the end ring section were not excessively large compared to that of the total cross sectional area of thezigzag conductor. How ever, in view of the teachings of this application, it will be evident that radial vents similar to 36 in FIGURE ,6

can be readily added to the arrangement of FIGURE 7 to adapt it to the centrifugal casting method.

It is generally desirable to utilize only one or.

The arrows I The design of FIGURE 8 is made to serve the samepurpose as the construction illustrated in FIGURE 7. As shown, a zigzag arrangement for the winding is used but a relatively long conductor sectionextends axially of ,the rotor andis locatedin themiddle portionsthereof.

Two different types. of laminations 1321and7134- are used in providing the construction shown;

equally spaced from each other and from the vertical axis of the lamination while laminations 134,are.provided' with similar openings except thatthey are spaced inwardly from the openings which are used for. iproviding the 2 axially extending slot 139.;

In some instances the punchings delaminate. slightly 7 near the ends ofthe rotor. when it is removed from the molds after casting. In order to reduce, this undesirable effect, a precautionary measure can be resorted to which consists of locating either a temporary or permanently mounted ring 136 .on opposite ends of the punchingstack.

It=may be welded or otherwise sceuredtothe punching stack before casting. a The use of sucha ring exerts an axial compressiveforce on thelaminationgand also resists the tendency of the aluminum end ring'to decrease:

in diameter as it coo1s,-so .that parting of the laminations at the outer surface is less likely. to occur, particularly in those cases when the aluminum is still hot and me very flexible state;

The, advantagederived from practicing a roasting methodof the type disclosed herein is that it is'possible. to obtain high resistance rotors having high conductivity alloys. A high value of heat capacity can be maintained.

in the rotor .by employing larger quantities of the conductor material than thatwhichhas been possible by other. methods and constructions. Larger conductor and neck sections may be iusedwhich result in lower current densities and the provision of a zigzag current path exposes the larger surfaces of the conductor material to a likely to occur.

In view of the above, it will be evident that many modi-- fications and variations are possible in light of theabove teachings. It therefore is to be understood that within the scope of theiappended claims. the invention may be practiced otherwise thanas specifically described I What we claim asnew and desire to secure by Letters Patent of, the United States is:

The process of making. an induction motor rotor with a high resistance .cast squirrel, cage winding, said process comprising:

(a) stacking .contiguo-usly aplurality ofannulanmagnetic' laminations assembled-in sectionsto provide. a rotor core having a central bore, aligned apertures formed ,in the laminations ,of, said: sections ,com-' i municatingbetween adjacent sections and establishing a plurality of continuous passages by varying radial dimension through the: rotor .core, and vent openingsformed in the laminations of selected ones, i

of said sectionscommunicating from'said passages into said bore; (b) positioning an:upper and a lower casting mold at corresponding endsof said core, each'of said molds including end ring cavities communicating, with respective ends of each of said passages and gates interconnecting said cavities and said bore, said upper Laminations 132"; are merely equipped with a plurality of circular openings 9 mold having an aperture formed therethrough aligned with said bore and a vent opening in the top of said upper mold; (c) rotating said core and molds about an axis through said bore;

((1) pouring conductive molten metal through the aperture in said upper mold into said bore; and, (e) venting air displaced by the rising molten metal axially inwardly to the bore from said passages through said vent openings in the laminations, whereby trapping of air in said passages is avoided and molten metal is permitted to completely fill said passages.

l 0 References Cited by the Examiner UNITED STATES PATENTS 1,190,009 7/1916 Reist et al. 5 2,368,295 1/1945 Goran 22-203 2,996,773 8/ 1961 Summers 22-200.5

FOREIGN PATENTS 534,071 12/1921 France.

10 I. SPENCER OVERHOLSER, Primary Examiner.

MARCUS U. LYONS, Examiner.

E. MAR, Assistant Examiner. 

